Flotation process for removal of precipitates from electrochemical chromate reduction unit

Abstract

This invention is an improved form of a conventional electrochemical process for removing hexavalent chromium or other metal-ion contaminants from cooling-tower blowdown water. In the conventional process, the contaminant is reduced and precipitated at an iron anode, thus forming a mixed precipitate of iron and chromium hydroxides, while hydrogen being evolved copiously at a cathode is vented from the electrochemical cell. In the conventional process, subsequent separation of the fine precipitate has proved to be difficult and inefficient. In accordance with this invention, the electrochemical operation is conducted in a novel manner permitting a much more efficient and less expensive precipitate-recovery operation. That is, the electrochemical operation is conducted under an evolved-hydrogen partial pressure exceeding atmospheric pressure. As a result, most of the evolved hydrogen is entrained as bubbles in the blowdown in the cell. The resulting hydrogen-rich blowdown is introduced to a vented chamber, where the entrained hydrogen combines with the precipitate to form a froth which can be separated by conventional techniques. In addition to the hydrogen, two materials present in most blowdown act as flotation promoters for the precipitate. These are (1) air, with which the blowdown water becomes saturated in the course of normal cooling-tower operation, and (2) surfactants which commonly are added to cooling-tower recirculating-water systems to inhibit the growth of certain organisms or prevent the deposition of insoluble particulates.

What is claimed is: 
     
       1. An improved process for the electrochemical removal of a metal-ion contaminant from the aqueous blowdown from a cooling tower comprising: a. introducing said blowdown to an electrolytic cell including an anode having a surface of a material selected from the group consisting of iron, iron alloys, and insoluble compounds of iron and including a cathode;   b. passing through said blowdown in said cell an electric current established between said anode and cathode to form a suspension of a mixed precipitate of iron hydroxide and a hydroxide of said contaminant while evolving gaseous hydrogen at said cathode;   c. establishing above said blowdown in said cell a partial pressure of hydrogen so evolved exceeding atmospheric pressure, thus entraining most of the evolved hydrogen as bubbles in said blowdown in said cell;   d. introducing the resulting hydrogen-rich blowdown containing said suspension into a vented chamber, whereby the hydrogen so entrained combines with particles of said precipitate to form a precipitate-containing froth; and   e. separating said froth from said blowdown.   
     
     
       2. The process of claim 1 wherein said partial pressure is established by retaining within said cell all of the evolved hydrogen accumulating above the blowdown therein. 
     
     
       3. The process of claim 1 wherein said partial pressure is established by releasing from said cell a selected fraction of the evolved hydrogen accumulating above the blowdown therein. 
     
     
       4. The process of claim 1 wherein a flotation promoter for said precipitate is introduced to said vented chamber. 
     
     
       5. An improved method for the electrochemical removal of hexavalent chromium contaminant from the blowdown from a cooling tower incorporated in a recirculating-water system, said blowdown containing a surfactant added to the recirculating-water system to promote efficient operation thereof, said blowdown being saturated with air as a result of having been passed through said cooling-tower, comprising: a. introducing said blowdown to an electrolytic cell including an anode having a surface of a material selected from the group consisting of iron, iron alloys, and insoluble compounds of iron and including a cathode;   b. passing through said blowdown in said cell an electric current established between said anode and cathode to form a suspension of a mixed precipitate of iron hydroxide and chromium hydroxide while evolving gaseous hydrogen at said cathode;   c. establishing above said blowdown in said cell a partial pressure of hydrogen so evolved exceeding atmospheric pressure, thus entraining most of the evolved hydrogen as bubbles in said blowdown in said cell;   d. introducing the resulting hydrogen-rich blowdown containing said suspension and said surfactant into a vented chamber, whereby the hydrogen so entrained and air entrained in the introduced blowdown combine with particles of said precipitate to form a chromic-hydroxide-containing froth; and   e. separating said froth from said blowdown.   
     
     
       6. The process of claim 5 wherein said partial pressure is established by retaining within said cell all of the evolved hydrogen accumulating above the blowdown therein. 
     
     
       7. The process of claim 5 wherein said partial pressure is established by releasing from said cell a selected fraction of the evolved hydrogen accumulating above the blowdown therein. 
     
     
       8. The process of claim 5 wherein a flotation promoter in addition to said surfactant is introduced to said vented chamber.